By: Morgan Evans, ChE Project Engineer, Americhem System Inc.
The design of suction piping for pumps is often overlooked and may seem insignificant at times but it’s this design that plays a vital role in determining pump performance and heath. Reason being that the design of suction piping defines the resulting hydraulic conditions experienced at the pump inlet or impeller. Many problems and failures can be traced back to the design of suction piping failing to produce a uniform velocity distribution profile. These problems include, but are not limited to :
- Noisy operation, turbulence, and friction losses
- Random axial load oscillations
- Premature bearing or seal failure
- Insufficient fluid pressure leading to cavitation
- Occasional damage on the discharge side due to liquid separation
Any one of these problems can result in pump suction failure. For these reasons it is important to include minimum straight length requirements, the correct nominal pipe size, and the appropriate fittings in the design of suction piping for pumps to mitigate these issues.
What is Cavitation?
Cavitation is the formation of vapor cavities in a liquid that are the result of forces acting upon the liquid . While cavitation is possible to occur in any type of pump, they are most common in centrifugal pumps where bubbles quickly develop around impeller axes. These bubbles can come from air pockets in the suction piping or from an insufficient Net Positive Suction Head available (NPSHa). An insufficient NPSHa causes the liquid to turn to vapor and form bubbles at low pressures. In any form, these vapor bubbles then pass from the middle section of the impellers to the outer edges where the centrifugal forces create higher pressures. These higher pressures cause bubbles to quickly collapse or implode with great force. In short, pump cavitation is the rapid succession and released energy of the implosion of gaseous cavities which cause intense rattling sensation that can damage a pump .
Minimum Straight Length Requirements
Pumps, especially centrifugal pumps, work best with a smooth, laminar flow. Higher liquid velocities and higher turbulence in pump suction increase the chance of cavitation occurring in the pump. To minimize higher liquid velocities and turbulence a general rule of thumb is applied. The rule of thumb is to keep a minimum straight length of pipe going into a pump equal to 5 to 10 times the diameter of the pump nozzles. That is, if your pump suction is 2 inches the minimum straight length should be 10-20 inches of pipe. To further understand this concept, a straight length of pipe needs to be defined. The American National Standards Institute (ANSI) defines that the minimum straight length of pipe for the pump intake is the minimum defined distance that has no flow disturbing fittings. These flow disturbing fittings are things such as partially open valves, tees, short radium elbows, etc. . Flow disturbing fittings do not include reducers; which are commonly used in suction side piping. These fittings are commonly found on pump suctions due to the nominal pipe size of the suction being larger than that of the pump nozzle.
Pump Suction Pipe Size
It is often very common for pump nozzles to be different sizes than that of the suction/discharge piping of the system. A frequent practice is to employ suction side piping one to two nominal pipe sizes larger than that of the pump nozzle. To understand why this is commonly practiced Darcy’s equation shown in Equation 1  can be used for explanation.
head loss due to friction = h𝑓 = 𝑓Lv2 / 2gD (1)
Where: h𝑓 = friction head loss 𝑓= Darcy resistance factor L= length v= mean velocity of fluid g= acceleration due to gravity D= pipe diameter
It is shown in Equation 1 that one method to reduce the head loss due to friction is to increase the diameter of the cross sectional area. In other words, by increasing the diameter of suction piping to a centrifugal pump you decrease the head loss due to friction thus increasing the total head pressure to the pump suction and reducing chance of cavitation. This method of fluid flow optimization in the suction piping of centrifugal pumps is what calls for using reducer fittings to adapt to the smaller diameter pump nozzles.
Appropriate Pipe Fittings
In many piping systems, whether industrial, commercial, or residential, pipe fittings called reducers are often used. There are two types of reducers, concentric and eccentric. Eccentric reducers are called “eccentric” because they are not symmetrical. That is, the diameter on both sides of the fitting are not the same and have mis-matched center lines (centerlines for pipe on either side of the fitting are different). Eccentric reducers are often used in the suction piping of centrifugal pumps due to these pumps often having smaller nozzle sizes than that of the suction piping. Eccentric reducers are used over concentric reducers in the instance of centrifugal pumps because the eccentric shape changes the direction of flow and helps prevent the buildup of air or vapor pockets in high points of the pipe which can cause cavitation and damage to the pump.
For the same reason the appropriate fittings need to be used, the orientation of eccentric reducers is fundamental for proper pump function. If the source of supply is above the pump, then the eccentric reducer must be placed with the flat side on the bottom of the pipe. If the source of supply is below the pump the reducer must be orientated with the flat side on the top of the pipe. In the case of long
horizontal pipe runs, air pockets are avoided by installing the eccentric reducer with the flat side up . These orientations are illustrated in Figure 1.
The design of suction piping for all pumps is critical for the performance and health of the pump. Failure to properly design suction piping can lead to several issues such as noisy operation, turbulence, head loss due to friction, premature bearing or seal failure, and of course cavitation. Such issues can be mitigated by implementing proper design practices on suction piping such as including minimum straight length requirements, the correct nominal pipe sizes, and the appropriate fittings with correct orientations.
Figure 1: If the source of supply is above the pump, then the eccentric reducer must be placed with the flat side on the bottom of the pipe. If the source of supply is below the pump the reducer must be orientated with the flat side on the top of the pipe. In the case of long horizontal pipe runs, air pockets are avoided by installing the eccentric reducer with the flat side up .
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